1. Field of the Invention
The present invention is directed to a process or method for the production of single-material light-guiding fibers and to a crucible used in the method.
2. Prior Art
In the production of optical light-guiding fibers, the chemical composition of the glasses which are used are often altered in order to influence the path of the index of refraction between the core and the sheathing of the light-guiding fiber. Light-guiding fibers are known in which the index of refraction increases stepwise at the junction or interface between the sheath and the core so that the core has a higher index of refraction than the sheath or fiber cladding. Light-guiding fibers are also known in which the index of refraction increases continuously when preceding from the exterior surface of the fiber toward the interior. As a result of the differing chemical compositions, it is often difficult to match or coordinate the thermal and mechanical characteristics of the materials of the fiber core to the materials of the fiber sheath or cladding. For this reason, the long-term stability and the mechanical loadability of the cables formed of such light fibers can be disadvantageously influenced.
A single-material light-guiding fiber, which is formed from a single type of glass, was suggested in an article by J. P. Kaiser, E. A. J. Marcatilli and S. E. Miller, Bell System Technical Journal, Vol. 52(1973), pp. 265-269. A typical cross section of such a fiber is illustrated by a fiber generally indicated at 5 in FIG. 1 of the present application. The fiber 5 consists of a glass sheath 1, which surrounds a core 2, which does not touch the sheath at any point and is connected to the sheath 1 by glass strips or webs 3 and 4 which are as thin as possible.
Since only a single kind or type of glass is used in a single-material light-guiding fiber of this type, the above mentioned problems do not occur. Additionally, because of its hollow structure, this light-guiding fiber has the advantage of being capable of being heavily mechanically loaded. In addition, because of the use of only one kind or type of glass for the entire fiber 5, special glasses can be selected which glasses distinguish themselves, for example by high mechanical, chemical and thermal stability. In this context, it should be noted that due to using the same glass for both the core and sheath, the difficulties of matching this kind or type of special glass to a second kind or type of glass does not occur.
Since total reflection will occur at the boundary surface between the core 2 of the fiber and the ambient air or, respectively, ambient gas which is entrapped between the core 2 and sheath 1, the light conveyed in the core 2 cannot laterally leave the core.
Previously, the production of a single-material light-guiding fiber was very difficult. In the previous attempts of forming the fiber, a workpiece or blank was formed by providing a quartz rod on which two thin quartz plates were attached with an 180.degree. offset by being fused or melted on the surface of the rod. This rod was then inserted into a quartz tube and the thin quartz plates were then melted or fused to the interior surface of the quartz tube so that the quartz rod was supported in the quartz tube in a contilever manner by the quartz plates. After heating the blank to a temperature of about 1800.degree. C. to 2000.degree. C., the blank consisting of the quartz rod and the quartz tube can be drawn out into a fiber. With this type of production, a difficulty occurs with the securing of the thin as possible quartz plates onto the quartz rod and in addition, all parts must be thoroughly cleansed, particularly, the surfaces of the quartz rod. Contamination on the surface of the quartz rod will lead to the contamination of the subsequent core fiber and it is noted that even the tinest or smallest dust particle or the absorption of a moisture film by the quartz rod can lead to high scattering losses or absorption losses, respectively. In addition, this type of production process is not applicable for large scale or volume production.